The present invention relates to the field of hydrostatic units such as transmissions, pumps and motors. More particularly, this invention relates to means for reducing the oil volume of pistons slidably mounted in the cylinder block bores of hydrostatic units.
The oil volume in each piston bore is compressed to the operating pressure of the hydrostatic unit at some time during each rotation of the cylinder block. The fact that oil is compressible and takes energy to compress results in energy losses for units that do not have solid or sealed pistons. It is known in the hydrostatic art to utilize solid pistons to improve efficiency. However, solid pistons are relatively heavy, which reduces the maximum speed at which they can operate due to higher block tipping forces and higher centrifugal forces that cause piston burn as the pistons reciprocate longitudinally in the piston bores.
It is also known in the hydrostatic art that the oil-containing volume of a piston can be reduced by forming the piston with a hollow or cavity therein, then sealing the cavity so oil cannot enter. This is conventionally done by welding a cap member on the hollow piston body. Unfortunately, such welded pistons are generally costly to produce. Direct displacement (non-servo) units typically do not utilize pistons with conventionally reduced oil-containing volume because of the higher cost. Another problem with hollow welded pistons lies in the variation in control moments that occurs with changes in the rotational speed of the cylinder block. Since direct displacement units do not have a servo to control the swashplate, the operator feels the control moments to a greater degree and therefore experiences greater operator fatigue.
Therefore, a primary objective of the present invention is the provision of a reduced oil volume piston and cylinder block assembly that improves the efficiency of a hydrostatic unit without unduly increasing its cost.
Another objective of the present invention is the provision of a piston and cylinder block assembly wherein the filler material for the piston is retained in the cylinder block, rather than in the piston, so that the filler material has no impact on the centrifugal forces on the piston and causes no additional block tipping forces.
Another objective of the present invention is the provision of a reduced volume piston and cylinder block assembly that is economical to produce, as well as reliable and durable in use.
These and other objectives will be apparent to one skilled in the art from the drawings, as well as from the following description and claims.
The present invention relates to piston and cylinder block assemblies for hydrostatic units. These assemblies reduce the oil volume of the pistons. The reduced oil volume piston and cylinder block assembly includes a cylinder block with a central bore and a plurality of cylindrical piston bores radially spaced from the central bore, a plurality of pistons having one hollow end slidably mounted in the piston bores, and a plurality of stems correspondingly disposed in the piston bores and extending into the cavity at the hollow end of the pistons so as to displace or reduce the oil-containing capacity or volume of the piston.
The stem of this invention is formed separately from the piston body, as described below in three different embodiments. In the first embodiment, the stem is a separate component that is inserted in each piston bore. In the second embodiment, the stems are cast as an integral part of the cylinder block. In the third embodiment, the stems extend into the pistons because the stems are attached to a ring that is fixed to the bottom of the cylinder block.